[ruby/irb] Fix warnings in test_cmd.rb

Fixes this warning:

    warning: assigned but unused variable - err

https://github.com/ruby/irb/commit/298fcb57a3

test/irb/test_cmd.rb

@@ -416,7 +416,7 @@ module TestIRB
         ])
       IRB.conf[:PROMPT_MODE] = :SIMPLE
       irb = IRB::Irb.new(IRB::WorkSpace.new(self), input)
-      out, err = capture_output do
+      out, _ = capture_output do
         irb.eval_input
       end
 
@@ -437,7 +437,7 @@ module TestIRB
         ])
       IRB.conf[:PROMPT_MODE] = :SIMPLE
       irb = IRB::Irb.new(IRB::WorkSpace.new(self), input)
-      out, err = capture_output do
+      out, _ = capture_output do
         IRB::TestHelper.without_rdoc do
           irb.eval_input
         end